Receiver apparatus

ABSTRACT

A receiver ( 11 ) converts a received signal directly to a baseband signal, and includes: a switched-capacitor filter ( 19 ) controlling a cutoff frequency when the baseband signal is filtered according to a control signal provided for a switched-capacitor element ( 27 ); an oscillator generating a periodic signal; and a divider ( 31 ) dividing a periodic signal generated by the oscillator according to the received signal, and an output signal from the divider ( 31 ) is provided as the control signal for the switched-capacitor element ( 27 ).

TECHNICAL FIELD

The present invention relates to a receiver for receiving various signalbands such as the signal bands of a mobile telephone, a radio, etc.

BACKGROUND ART

Generally, a system for receiving various radio bands of signals such asa mobile telephone, a radio, etc. can be a superheterodyne system, adirect conversion system, etc. In the superheterodyne system amongvarious systems, a received signal is temporarily converted to a signalof an intermediate frequency, and then converted to a baseband signal.

When signals of various frequency bands are received in a receiver usingthe superheterodyne system, it is necessary for the receiver to processa broadband signal for signal processing of an intermediate frequencysignal depending on the received signal.

That is, for example, in the case of a receiver in the superheterodynesystem for receiving an AM signal and an FM signal in Japan, thereceiver requires a band pass filter for a broad signal bands to passthrough the two intermediate frequency band for receiving both AM and FMsignals. The receiver for processing these plurality of intermediatefrequency signals has the problem of a complicated configuration and alarge size.

A direct conversion system has been well known as a receiving systemhaving a simple configuration and a small size.

The direct conversion system converts a received signal directly to abaseband signal by mixing a received signal with a signal having thesame frequency as the received signal. The receiver in the directconversion system does not use an intermediate frequency, but converts areceived signal directly to a baseband signal, thereby requiring nofilter for removing an image signal for use in an RF (radio frequency)circuit unit, and realizing a small receiver. Thus, the directconversion system has attracted attention as a receiving system capableof realizing a small receiver.

However, when received signals of various frequency bands are received,the receiver in the direct conversion system, the receiver has toprocess signals of a broad band to process a baseband signal dependingon the received signal.

That is, in the receiver in the conventional direct conversion system,it has been necessary to prepare a filter for removing an unnecessarysignal for each signal band depending on each signal band.

It is necessary to switch these filters depending on each signal band.The control circuit for switching the filters has a complicatedconfiguration with an increasing number of signal bands, thereby causinga large receiver.

Additionally, a conventional receiver configures a filter by a passiveelement of a resistor and a capacitor, and has a problem of largevariance of filter characteristic.

Thus, the present invention has been developed to solve theabove-mentioned problems, and aims at providing a receiver capable ofbeing easily applied to various signal bands, and suitable forsemiconductor integration.

DISCLOSURE OF INVENTION

The receiver according to the first aspect of the present invention is areceiver for converting a received signal directly to a baseband signal,and includes: a switched-capacitor filter for controlling the cutofffrequency when the baseband signal is filtered according to the controlsignal provided for a switched-capacitor element, an oscillator forgenerating a periodic signal, and a divider for dividing a periodicsignal generated by the oscillator according to the received signal. Anoutput signal from the divider is provided as a control signal for theswitched-capacitor element.

With the above-mentioned configuration, a switched-capacitor filter isused as a frequency filter for passing a baseband signal. Therefore,various received signal bands can be supported only by varying thecutoff frequency of the switched-capacitor filter, thereby removing thefiltering process for each receiving band. Thus, a smaller receiver canbe realized.

In the receiver according to the second aspect of the present invention,the first divider is a programmable counter and can be configured by adivider in the system of a division to an integral multiple or thefractional-N system.

With the above-mentioned configuration, an arbitrary cutoff frequencycan be set, and various receiving bands can be supported.

In the receiver according to the third aspect of the present invention,the switched-capacitor filter includes at least an amplifier, and aresistor element as a feedback resistor of the amplifier can be realizedby the switched-capacitor element.

With the above-mentioned configuration, the operation and the effectsimilar to those of the first aspect can be obtained.

The receiver according to the fourth aspect of the present invention isa receiver for converting a received signal directly to a basebandsignal, and includes an oscillator for generating a periodic signal, amixer for mixing a periodic signal generated by the oscillator with thereceived signal, and outputting a baseband signal, a switched-capacitorfilter for controlling the cutoff frequency when filtering the basebandsignal output from the mixer according to the control signal providedfor the switched-capacitor element, and a divider for dividing aperiodic signal generated by the oscillator according to the receivedsignal, and the output signal from the divider is provided as thecontrol signal for the switched-capacitor element.

With the above-mentioned configuration, the signal output from thevoltage control oscillator is divided by a divider such as aprogrammable counter, etc., and the passband of the switched-capacitorfilter is varied using the divided signal. Therefore, a circuit forgenerating a reference frequency signal required to vary the passband ofa switched-capacitor filter can be omitted, thereby realizing a smallerreceiver.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will become more apparent by reference to thefollowing detailed description of the invention taken in conjunctionwith the accompanying drawings, wherein:

FIG. 1 shows a receiver according to the present invention;

FIG. 2A shows the configuration of the circuit of the switched-capacitorfilter;

FIG. 2B shows the relationship between the cutoff frequency in theswitched-capacitor filter and the resistance of the feedback resistor inthe primary integral active LPF;

FIG. 2C shows the switched-capacitor element;

FIG. 3A shows the control operation of the switching operation of theswitched-capacitor element;

FIG. 3B shows the divider in the fractional-N system; and

FIG. 4 shows the configuration of the synthesizer in the PLL system forvarying the frequency of an oscillation signal output from the localoscillator.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the present invention are described below byreferring to the attached drawings.

FIG. 1 shows the receiver of the present invention.

FIG. 1 shows a direct conversion receiver 11, an antenna 12, a band passfilter 13, a high-frequency signal amplifier 14, a mixer 15, a 90°phase-shifter 16, a local oscillator 17, an anti-aliasing filter 18, aswitched-capacitor filter 19, a baseband signal amplifier 20, an A/Dconverter 21, a signal processing unit 22, and a control signalgenerator 23.

The signal processing unit 22 is shown as one function block, andperforms various processes (for example, a detecting process, a digitalfilter processing, etc.) after conversion of a received signal to abaseband signal.

In FIG. 1, when a received signal is converted to a baseband signal andthe subsequent processes are performed with an analog signal, the A/Dconverter 21 is omitted.

The switched-capacitor filter 19 has the function of a low pass filterfor removing the high-frequency element of the baseband signal, and thecutoff frequency can be varied depending on the control signal outputfrom the control signal generator 23.

The direct conversion receiver 11 can be integrated on one chip.

The operations of the direct conversion receiver 11 are explained below.

First, when the direct conversion receiver 11 receives a received signalfrom the antenna 12, it removes an unnecessary signal using the bandpass filter 13, and amplifies the received signal using thehigh-frequency signal amplifier 14.

The amplified received signal is converted to a two orthogonal signalhaving phases different by 90° by the mixer 15, the 90° phase-shifter16, and the local oscillator 17. The signal input from the localoscillator 17 to the mixer 15 has the same frequency as the receivedsignal.

An excess signal is removed from the two orthogonal signals by theanti-aliasing filter 18 to protect them against the folding noisegenerated in the subsequent process, and the signals are then inputtedto the switched-capacitor filter 19.

The high-frequency element is removed by the switched-capacitor filter19 from the signals input to the switched-capacitor filter 19, and theresultant signals are amplified by the baseband signal amplifier 20.

The signal amplified by the baseband signal amplifier 20 is converted toa digital signal by the A/D converter 21, and is treated in the signalprocessing unit 22 by a predetermined digital filter processing such asa detecting process, etc.

When a received signal is converted directly to a baseband signal, thedirect conversion receiver 11 removes the unnecessary signal (an imagesignal, etc.) generated in the received signal by a low pass filter.

At this time, it is necessary to change the passband of the low passfilter depending on the signal band of the received signal. In thedirect conversion receiver 11 of the present embodiment, theswitched-capacitor filter 19 is used as a low pass filter to change thepassband for each signal band.

Thus, as a conventional receiver, a filter is prepared for each signalband, it is not necessary to perform the process of switching a filterdepending on the signal band of a received signal, and a simple circuitconfiguration can be realized.

That is, one switched-capacitor filter 19 can process a received signalof a desired signal band among a plurality of signal bands by providingthe switched-capacitor filter 19 having the low pass filter function andthe function of changing a cutoff frequency in the direct conversionreceiver 11. Thus, the direct conversion receiver 11 can be downsized.

Described below is the circuit configuration of the switched-capacitorfilter 19.

FIG. 2A shows the circuit configuration of the switched-capacitor filter19.

In FIG. 2A, the switched-capacitor filter 19 is a well-knownstatus-variable active LPF (low pass filter) (or a bi-quad low passfilter).

In the switched-capacitor filter 19, an integrator 25 and an inverseamplifier 26 are added to a secondary bi-quad active LPF 24 in whichresistors are connected in series to the input terminal of the op-amp,thereby configuring a closed loop. The output of the integrator 25 ofthe switched-capacitor filter 19 has the function similar to that of theoutput of the low pass filter of the conventional receiver.

Normally, the 3 dB falling-pass bandwidth ω in the switched-capacitorfilter 19 holds the following equation.ω=1/RC−(1)

-   -   where R and C respectively indicate the resistance of a feedback        resistor and the capacitance of a capacitor of the secondary        bi-quad active LPF 24.

The switched-capacitor filter 19 raises the cutoff frequency fc when the3 dB falling-pass bandwidth ω increases, and reduces the cutofffrequency fc when the 3 dB falling-pass bandwidth ω decreases.

FIG. 2B shows the relationship between the cutoff frequency fc in theswitched-capacitor filter 19 and the resistance of the feedback resistorR in the primary integral active LPF 24.

As shown in FIG. 2B, when a high cutoff frequency fc is to be set, theresistance of the feedback resistor R in the secondary bi-quad activeLPF 24 is set small. When a low cutoff frequency fc is to be set, theresistance of the feedback resistor R in the primary integral active LPF24 is set large.

Thus, when the cutoff frequency fc of the switched-capacitor filter 19is varied, the resistance of the feedback resistor R of the secondarybi-quad active LPF 24 is varied.

Described below is the method for varying the resistance of the feedbackresistor R in the secondary bi-quad active LPF 24.

FIG. 2C shows a switched-capacitor element 27 used as a feedbackresistor R of the secondary bi-quad active LPF 24.

As shown in FIG. 2C, the switched-capacitor element 27 comprises acapacitor 28 and two switches T1 and T2, and the resistor element havingthe resistance depending on the control signal fo can be obtained byalternately switching the switches T1 and T2 connected to the capacitor28 using the control signal fo. The resistance RE of theswitched-capacitor element 27 is represented by RE=1/(f0·C).

The resistance of the switched-capacitor element 27 can be varied byreducing or raising the speed of the switching operation of the switchesT1 and T2.

Generally, the cutoff frequency fc of the filter formed by the capacitorCl and the resistor R1 is represented as follows.fc=1/(2πC 1 ·R 1)

When a common switched-capacitor filter is used, the following equationholds.fc=(f 0 ·C)/(2πC 1)

In the case of the switched-capacitor filter formed by two capacitors(capacitors C and Cl) on the same IC chip, the two capacitors have thecapacitance in the same direction and there is variance of capacitance.Therefore, the precision of the cutoff frequency is not high.

As a result, assuming that the variance coefficient of the capacitanceof the two capacitors is k, the cutoff frequency fc of theswitched-capacitor filter is represented as follows. $\begin{matrix}{{fc} = {\left( {{f0} \cdot k \cdot C} \right)/\left( {2\quad{\pi \cdot k \cdot {C1}}} \right)}} \\{= {\left( {{f0} \cdot C} \right)/\left( {2\quad\pi\quad{C1}} \right)}}\end{matrix}$

Thus, the variance of the capacitance between the capacitor C and thecapacitor Cl can be absorbed, and the precision of the control signal focan be enhanced.

Described below is the method for generating a control signal fo to beinput to the switches T1 and T2.

FIG. 3A is an explanatory view showing the method for generating thecontrol signal fo to be inputted to the switches T1 and T2.

FIG. 3A shows a programmable counter 31 for inputting the frequency fckof the input signal and outputting a control signal of the frequency fo1depending on the binary value Np1 (integral multiple). At this time, thecontrol signal output from the programmable counter 31 is output basedon the signal band of the received signal.

That is, the reference signal fck input to the programmable counter 31is divided into the signal of fo1=fck/Np1 depending on the signal bandof a received signal, and fo1 controls the switching operation of theswitched-capacitor filter 19 as a control signal.

For example, relating to the control signal fo1 output from theprogrammable counter 31, the switch T1 is closed (the switch T1 isturned ON) and the switch T2 is open (the switch T1 is turned OFF) whenthe control signal fo1 is at the H (high) level. Thus, the electriccharge is stored in the capacitor 28.

When the control signal fo1 is at the L (low) level, the switch T1 isopen (the switch T1 is turned OFF) and the switch T2 is closed (theswitch T2 is turned ON). thus, the charge stored in the capacitor 28 isdischarged to the switch T2.

By increasing the speed of the ON/OFF switching operation on theswitches T1 and T2, the resistance of the switched-capacitor element 27as the feedback resistor R is reduced.

On the other hand, by reducing the ON/OFF switching operation, theresistance of the switched-capacitor element 27 as the feedback resistorR is increased.

Described below is the case in which received signal of a differentsignal band is received.

The switched-capacitor filter 19 has to vary the cutoff frequency foreach different signal band. To set a lower cutoff frequency, thefrequency of the control signal fo input to the switched-capacitorelement 27 is reduced to increase the resistance R based on theabove-mentioned ω=1/RC−(1). At this time, the binary value for output ofthe control signal fo of a low frequency from the programmable counter31 is input to the programmable counter 31.

On the other hand, to set a higher cutoff frequency, the frequency ofthe control signal fo input to the switched-capacitor element 27 israised to reduce the resistance R based on the above-mentionedω=1/RC−(1). At this time, the binary value for output of the controlsignal fo of a high frequency from the programmable counter 31 is inputto the programmable counter 31.

Thus, by varying the speed of the ON/OFF switching operation on theswitches T1 and T2 according to the control signal fo1 output by theprogrammable counter 31, the resistance of the switched-capacitorelement 27 can be changed. By varying the resistance of theswitched-capacitor element 27, the cutoff frequency fc of theswitched-capacitor filter 19 can be varied.

FIG. 3B shows the divider of the fractional-N (fractional number)system.

In FIG. 3B, a divider 32 of the fractional-N system has a division valueincluding decimal places, and a desired division ratio, which is notlimited to 1/N, can be arbitrarily set.

The programmable counter 31 determines the division ratio depending onthe value of the integral multiple of the reference signal, but thedivider 32 in the fractional-N system can arbitrarily set the divisionratio by reducing or adding the pulse of the input reference signal fck.

The switching operation of the switched-capacitor element 27 iscontrolled by inputting the signal fo2 output from the divider 32 of thefractional-N system to the switched-capacitor element 27 as a controlsignal. Thus, by controlling the switching operation of theswitched-capacitor element 27 using the signal output by the divider 32of the fractional-N system, the control can be performed more preciselythan by the programmable counter 31.

Thus, in the direct conversion receiver 11, the received signal ofvarious signal bands can be received with a simpler configuration byusing the switched-capacitor filter 19 as a low pass filter for removingan unnecessary signal of a baseband signal.

Since the switched-capacitor filter 19 can be generated in asemiconductor integrated circuit, the entire circuit can be downsized.

Furthermore, to generate a control signal for varying the cutofffrequency, the programmable counter 31 and the divider 32 of thefractional-N system are adopted and the data value is changed to changethe division ratio of them, thereby easily changing the cutofffrequency.

The direct conversion receiver 11 according to the present embodiment isnot limited to the above-mentioned examples.

For example, an output signal from the local oscillator 17 is divided,and the divided signal can be used as a control signal fo for varyingthe cutoff frequency of the switched-capacitor filter 19.

FIG. 4 shows the configuration of a synthesizer 41 of the PLL (phaselocked loop) system for varying the frequency of an oscillation signalof the local oscillator 17 in the direct conversion receiver 11.

FIG. 4 shows a voltage controlled oscillator (VOC) 42, a programmablecounter 43 for dividing into an integral submultiple the frequency of asignal input from the voltage controlled oscillator 42 depending on theinput binary value (integral multiple), a phase comparator 44 forcomparing the signal output from the programmable counter 43 with thereference signal fx and outputting a voltage value depending on thephase difference, and a low pass filter 45 for removing an unnecessaryvoltage element from the voltage value output from the phase comparator44, and generals a DC control voltage.

A programmable counter 46 divides a frequency of a signal output fromthe voltage controlled oscillator 42 into 1/P. A divider 47 fixedlydivides into 1/N the frequency of the reference signal fx output fromthe crystal oscillator, etc. The reference signal fr output from thedivider 47 is set to fr=fx/N.

The synthesizer 41 shown in FIG. 4 is a synthesizer of the PLL systemusing the well-known programmable counter 43.

The phase comparator 44 compares relating to the phases the referencesignal fr output from the divider 47 with the signal fw obtained bydividing the programmable counter 43 into 1/k the signal fv output fromthe voltage controlled oscillator 42, and outputs the voltage dependingon the phase difference. Thus, the signal fv output from the voltagecontrolled oscillator 42 maintains the relation of fv=fr·K by the PLLloop of the synthesizer 41.

The signal fv output from the voltage controlled oscillator 42 isdivided by the programmable counter 46 into 1/P, and the divided signalis used in controlling the ON/OFF switching operation on the switches T1and T2 of the switched-capacitor element 27 in the switched-capacitorfilter 19.

At this time, since the control signal output from the programmablecounter 46 can also be used as an oscillation signal for conversion of areceived signal to a baseband signal, the binary value input to theprogrammable counter 46 holds a predetermined relationship with thereceived signal.

The programmable counter 43 or the programmable counter 46 can beconfigured as a divider of the fractional-N system.

Thus, when the programmable counter 46 is a divider of the fractional-Nsystem, the frequency of the control signal input to theswitched-capacitor filter 19 can be arbitrarily set.

According to the receiver of the present invention, a switched-capacitorfilter is used as a low pass filter of a receiver for converting areceived signal directly to a baseband signal, and the cutoff frequencyis changed. Thus, the cutoff frequency can be changed, thereby realizinga receiver of a simple configuration with the capability for varioussignal bands. Additionally, by using a programmable counter of thesystem of dividing by an integral multiple as a divider or of thefractional-N system, the cutoff frequency of the switched-capacitorfilter can be set to an arbitrary frequency.

1. A receiver which converts a received signal directly to a basebandsignal, comprising: a switched-capacitor filter controlling a cutofffrequency when the baseband signal is filtered according to a controlsignal provided for a switched-capacitor element; an oscillatorgenerating a periodic signal; and a divider dividing a periodic signalgenerated by said oscillator according to the received signal,characterized in that an output signal from the divider is provided asthe control signal for the switched-capacitor element.
 2. The receiveraccording to claim 1, characterized in that said divider is aprogrammable counter and is a divider in a system of a division to anintegral multiple or a fractional-N system.
 3. The receiver according toclaim 1, characterized in that said switched-capacitor filter comprisesat least an amplifier, and a resistor element, which functions as afeedback resistor of the amplifier, is realized by theswitched-capacitor element.
 4. A receiver which converts a receivedsignal directly to a baseband signal, comprising: an oscillatorgenerating a periodic signal; a mixer for mixing a periodic signalgenerated by said oscillator with the received signal, and outputting abaseband signal; a switched-capacitor filter controlling a cutofffrequency when filtering the baseband signal output from said mixeraccording to a control signal provided for a switched-capacitor element;and a divider dividing a periodic signal generated by said oscillatoraccording to the received signal, characterized in that the outputsignal from said divider is provided as the control signal for theswitched-capacitor element.